Radioimmunotherapy (RIT) provides an opportunity to deliver more specific radiation to tumor cells while sparing normal tissue. This new medical technology uses millions of cancer seeking antibodies to guide radiation to the cancer. The radiation is conjoined to the antibodies, accompanying them as they flow through the bloodstream. When the antibodies arrive at the cancer site, they attach and remain there, giving their radioactive counterparts the opportunity to destroy the cancer cells. Clinical trials have shown RIT to be successful in treating leukemia and lymphoma, and studies have opened the way for testing these methods on a wide range of cancers. The antibodies used in RIT are monoclonal antibodies (MAbs). In RIT planning, the MAbs are modified to bind to radioactive metals (e.g., Indium-111), which can be visualized with a gamma camera in nuclear medicine imaging. Images from the gamma camera show areas where the Mabs localize in the body. In RIT therapy, the radioactive metal is switched to Yttrium-90, which delivers local radiation to the tumor. In RIT planning the conjugate-view planar imaging method is most widely used for radioactivity quantitation. However, photon scattering and superposition of overlying activities reduce the contrast of planar images. Uncertainty in activity quantitation therefore increases. Recent research using CT and MRI combined with planar imaging is encouraging, yet has drawbacks because the physiological uptake of antibodies may not correspond exactly to the anatomical configuration of an organ or tumor. This difficulty can be overcome by using SPECT. In this proposed research, alternating transmission/emission quantitative SPECT is suggested as a replacement for planar imaging. The proposed method will not increase acquisition time. It is hypothesized that quantitative SPECT will outperform planar imaging in terms of lesion detection and dose estimation in lesions and organs. Novel image reconstruction algorithms will be developed to provided quantitative SPECT images. Our hypothesis will be verified through human observer studies and Channelized Hotelling observer studies.